Urban Heat and Urban Landscapes

Local heat island phenomena, not always directly correlated with global warming-related events, are common endpoints for a host of interlocking processes. Important variables for assessing these local processes include the level of urban forestation, cited by Emily Nobel Maxwell as critical to evapotranspiration and cooling; the aggregate effects of site-specific microclimates, discussed with case studies by Pippa Brashear; and the repertoire of diverse plant species that have adapted to a city’s particular conditions, a special interest of Ed Toth. Cities need to keep all of these variables, and more, in mind when assessing the contribution of their landscapes to heat island effects.

Brian Stone’s comments at the outset of this panel included a key definition: “urban heat management lessens the risk of heat-related illness through land cover change and reduced waste heat emissions.” The sequence of terms may subtly indicate priorities: the primary concern, protecting human health, requires holistic intervention in landscapes as well as implementation of heat-reducing measures. Health depends on complex systems and feedback loops within entire environments, not just temperature-specific improvements. Reinforcing the importance of human health impacts, Reginald Blake cited observations by the New York City Panel on Climate Change that heat and humidity will combine to elevate mortality and morbidity over the coming decades. Though these burdens are systemic, they fall unevenly on different population segments. Considerations of environmental justice call for increased attention and research, he said, ideally culminating in high-resolution mapping of neighborhood-level vulnerabilities, including air quality and mold. Nobel Maxwell also pointed out that while flooding causes greater damage to infrastructure, “extreme heat is the number one weather- and natural disaster-related threat to human health in the country.”

The “Urban Heat and Urban Landscapes” panelists, with moderator James Russell.

After hearing several examples of manufactured replacements of natural systems leading to unsustainable urban heat island effects, moderator James Russell provocatively conjectured that perhaps “the suburbs have got it figured out: less of a heat island effect, lots more green cover.” Russell followed up by asking, “Should we actually not be thinking so hard about densification, and think of de-densification?” Stone emphatically rejected this inference on the grounds that “most environmental problems tend to be minimized with increased density,” including stormwater generation, energy consumption, relative emission levels, and the losses of species habitat and migratory corridors associated with sprawl. “The heat island,” he continued, “is the one exception.” In other words, the relative thermal comfort associated with low-density development comes at the cost of too many externalities in other areas.

Most environmental problems tend to be minimized with increased density. The heat island is the one exception, but the relative thermal comfort associated with low-density development comes at too high a cost in other areas.

While the panelists agreed that density is generally a positive step toward achieving a symbiotic relationship with nature, they also acknowledged that we are far from achieving that symbiosis. Viewing cities as components of wider natural systems rather than exceptions to them, Toth advocated a management approach rooted in the recognition “that nature is here, that functioning ecosystems are here, that biological processes are here, and that they don’t just stop existing because it’s highly urbanized.”

Cities are components of wider natural systems, rather than exceptions to them. The ecosystems that are here don’t stop existing because the region is now highly urbanized.

In keeping with the sentiment that even the most focused objectives and outcomes exist within a larger context, Brashear noted that discrete disasters provide opportunities to study long-range aspects of change and resilience. For example, the relation between acute events and chronic processes was particularly salient in the August 2003 northeastern power blackout, which was triggered, as Stone noted, by a heat wave.